Efforts are being directed towards the prevention or control of graft-versus-host disease in human allogeneic bone marrow transplantation. Since such graft-versus-host disease is mediated by alloreactive T cells in the inoculated marrow, reagents and techniques have been developed to remove these T cells from the marrow inoculum and to measure the success of that depletion. To this end, several murine monoclonal antibodies specific for antigens expressed on human T cells have been developed, three of which are cytotoxic. These antibodies have been untilized, in conjunction with other depletion techniques, for complement-mediated lysis of T cells in marrow. By a clonogenic assay now available, residual T cells in marrow following such a depletion are at a level of less than 0.01% of the total cell population. With respect to the control of alloreactive T cells mediating graft-versus-host disease, studies on the orgin or generation of such alloreactivity have been undertaken in murine radiation bone marrow chimeras. It has been shown that the generation is influenced by a unique interaction of T cell genotype and the T cell maturation environment. The fine specificity of human CTL has been demonstrated to be sufficient to distinguish among alpha 1 and alpha 2 domain changes of class I major histocompatibility comples molecules. Such mature alloreactive human cytotoxic T cells have been further studies with respect to cell surface molecules utilized in their interactions with target cells as the basis for therapeutic interventions with the intent of preventing tissue damage mediated by such alloreactive cytotoxic T cells.